Power amplifier

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Interior view of an audio power amplifier with switched-mode power supply for operation on the 12 V on-board network of passenger vehicles
High-frequency power triode 3CX1500A7

The output stage is the last electronically active (i.e. amplifying) stage of a power amplifier before the amplified signal reaches the load. The entire device or the “power amplifier” assembly is often referred to as an output stage.

On the other hand, the active components used to control the output stage components are sometimes referred to as “pre-stages”. The terms have historically evolved from the age of tube amplifiers , in which one or more tube systems can be assigned to each of these stages .

Classification

Selective power amplifiers

Selective power amplifiers are used for narrow frequency ranges (e.g. 3.60-3.62 MHz), preferably in transmitters , in order to generate high-frequency power to feed antennas . At least one selective filter, in the simplest case an oscillating circuit , is always used between the amplifier and the antenna in order to suppress harmonics . Therefore no (amplitude) linearity is required. These output stages are mostly operated in C mode in order to ensure a high efficiency of 80%. They are mainly used in transmitters and ultrasound transmitters .

LF broadband amplifier

LF broadband amplifiers (e.g. 10–50,000 Hz) are used to control loudspeakers , so-called audio amplifiers . In order to avoid unwanted harmonics (it is not filtered afterwards ), these (analog) output stages are always operated in the A or AB operating mode , thus achieving efficiencies between 20 and 70%. The main area of ​​application is electroacoustics.

RF broadband amplifier

HF broadband amplifiers (e.g. 5-860MHz) are preferably used as antenna amplifiers and in cable systems. In addition to the amplification, the maximum output power is crucial, at which the undesired mixed products that arise remain below a certain limit. The power is usually given in dBm . Typical values ​​are 20–70 dBm (100 mW - 10 kW). HF broadband amplifiers work mainly in A mode, for high output power also in AB mode.

Broadband amplifier for very wide frequency ranges

These broadband amplifiers, which are designed for very wide frequency ranges (z. B. 0-200 MHz), can be found u. a. in oscilloscopes . Such DC-coupled differential amplifiers enable a very wide frequency range from DC voltage to sometimes in the gigahertz range. They have a high input resistance (typically 20 MOhm || 10 pF) and have switchable input sensitivities. In tube oscilloscopes, these amplifiers supply the voltage for the deflection plates of the Braun tube .

Broadband amplifier

Broadband amplifiers can also be implemented as pulse-width-modulated switching amplifiers with a downstream LC low-pass filter. The low-pass filter is used to suppress the switching frequency of around 100–1000 kHz. The efficiency is often over 90%. Applications are audio amplifiers ( class D amplifiers in the kilowatt range) and, in a broader sense, frequency converters in drive technology with outputs up to the megawatt range and a frequency range of around 10–500 Hz.

Electronic switches

They are used in switch mode power supplies and switching regulators . They bring a medium to high output (a few watts up to the kilowatt range, in power electronics meanwhile also a few megawatts). Electronic switches have high switching speeds (typically greater than 10 A / ns), the output voltages are between 0.8 V and 5 kV.

Broadband amplifier for the low frequency range

Operating modes

Characteristic curve of an amplifier tube with the various operating points A, AB, B. Anode current I a as a function of the grid voltage U g .
Hysteresis curve of an output transformer ( magnetic saturation of the iron core)

Power amplifiers work in classic linear operation with low efficiency and are then classified according to the position of the operating point of the output stage:

  • A operation
  • B operation
  • AB operation

In addition, they can also work "digitally" with a high degree of efficiency:

  • C operation (operation on oscillating circuit, only at high frequency)
  • D amplifier (switching operation on LC low pass)
  • E-operation (switching operation with oscillating circuit)

A operation

There are four common circuits for so-called class A amplifiers:

  1. a voltage divider consisting of a controllable component (single ended) and a resistor,
  2. a voltage divider consisting of a controllable component and a coil, a transformer or the load itself,
  3. a voltage divider consisting of a controllable component and a current source (which is usually implemented by a further controllable component) and
  4. a voltage divider made up of two controllable components (push-pull) . The sum of the currents through both components is (largely) constant.

In all circuits, current always flows through all controllable (active) components - this is the feature that is described by the letter A. Since no component is ever completely blocked, i. H. becomes currentless, no so-called takeover distortions occur in principle.

Maximum efficiency, distortion behavior and component costs are different, the low efficiency is a disadvantage:

  • Depending on the model, theoretically maximum efficiency of 6.25% ( single ended with resistance), 25% ( single ended with power source or directly driven) or 50% ( push-pull ).
  • high quiescent current of 200% ( single ended with resistance), 100% ( single ended with current source or directly driven) or 50% ( push-pull ) of the simple peak current ( I p ).

In the single-ended A amplifier, the operating point is in the middle of the linear part of the characteristic. In the case of a tube amplifier, the grid voltage must not become positive, otherwise considerable distortion due to clipping will occur. It must be ensured that a collector or anode current flows at all times.

The transfer characteristic of an A amplifier, also as a push-pull amplifier, is similar to a J-shaped arc section. The Fourier transformation of this transfer function results in a dominance of the even harmonics.

AB operation and B operation

Advantages:

  • No matching transformer is required for ironless circuits or when using transistors
  • If transformers are used, no continuous direct current flows through the coils, which would premagnetize the iron core on one side. Therefore, distortions due to the curvature of the hysteresis curve can only arise with a very high modulation or an iron core that is too small
  • Without an input signal, low (AB) or negligible (B) power consumption
  • High performance bandwidth product
  • Good distribution of the power loss (waste heat) possible over several components

Disadvantage:

  • Only available as an "integrated circuit" (IC) for small outputs
  • Efficiency of around 60 to over 80%, depending on the circuit design (stepped operating voltage is common for high-performance output stages, see class H)
  • Symmetrical push-pull circuit required
  • B mode: Distortion (harmonic distortion) at low powers

The two operating modes differ in “one” detail: In B mode, the quiescent current is zero, in AB mode it is a few mA. Everything else is identical. Depending on the signal strength, one transistor only becomes more or less conductive with positive half-waves, the other only with negative half-waves of the input signal. So each only transmits half (electrical 180 degrees) of the signal. This arrangement is also called push-pull , since one transistor “pushes” current into the load and the other causes a current flow in the opposite direction, ie “pulls” it. In B operation, with very low signal voltages, it can happen that neither of the two transistors is conductive. Then crossover or deadband distortion occurs. This is avoided in the AB amplifier.

Push-pull output stage
Ironless output stage with complementary transistors Q4 and Q5

In the adjacent circuit diagram, the transistors Q4 and Q5 form the push-pull output stage with single-ended control and asymmetrical operating voltage. The upper transistor is of the NPN type and the lower is of the PNP type, with the components each having opposite electrical parameters. Diodes D1 and D2 provide the base bias to reduce crossover distortion when the transistors alternate when conducting. This operating mode of the transistors is also known as AB mode.

Commercial amplifiers
An AB push-pull output stage in a discrete design

Class AB power amplifiers are the most widely used power amplifiers in consumer electronics . They occur in an integrated design with medium power as IC (e.g. the hybrid STK types) or in more expensive amplifiers with discrete individual transistors. The picture shows a class AB push-pull output stage in a hi-fi amplifier. The output stage transistors can be seen under (1), which are driven in push-pull by the two driver transistors (2). The two capacitors under (3) serve as a buffer store to filter the symmetrical supply voltage (removal of the 100 Hz hum from the rectifier bridge ) and to provide enough current for short-term power peaks (bass). The integrated circuit under (4) is the source switch that is controlled by the microcontroller of the receiver / amplifier and with which the signal source is selected.

C operation

Advantages:

  • simple construction
  • no power consumption without an input signal
  • the power consumption increases roughly proportionally to the output power
  • high efficiency over 80%.

Disadvantage:

  • very high distortion (large distortion factor)
  • unsuitable for audio purposes.

This amplifier is mostly used in HF output stages for continuous transmissions. For carrierless transmissions ( single sideband modulation , SSB), amplifiers in C mode are not suitable due to the high level of distortion. The operating point is selected in such a way that no quiescent current flows even with a small modulation, which leads to a strong distortion of the output signal. This is irrelevant for frequency-modulated signals. The efficiency in C operation can be very high at up to 90%, so the power loss is low. This is an important property when you consider that a transmitter is often supplied with 100 kW and even more power. HF resonant circuits or pi filters must be used between the output stage and the antenna (load) in order to filter out unwanted harmonics .

D amplifier

Pulse width modulation to generate an approximately sinusoidal curve of the short-term mean value of the voltage - this voltage curve acts like a sinusoidal voltage on sluggish consumers

In a class D amplifier, the power transistors are controlled with the help of pulses of different lengths (PWM - pulse width modulation). They are therefore switched on and off at a high frequency (over 100 kHz) (switching amplifier) ​​and not, as in the other classes, operated linearly. This has the advantage that there is hardly any power loss at the transistors. This achieves a high degree of efficiency. During the switchover phase, non-negligible switchover losses occur, which increase linearly with increasing operating frequency. The switched output signal must be filtered with a low pass before it is passed on to the sound transducer . Otherwise, the speaker cables will act as an antenna for the RF switching frequency and emit strong electromagnetic interference that can affect other devices.

So that the circuit complexity for the subsequent low-pass filtering can be kept low, the switching frequency is far above the highest signal frequency. Typical switching frequencies of LF amplifiers in Class D operation are 768 kHz and 1536 kHz (8 × 96 kHz and 16 × 96 kHz). Output low-pass filters are simple LC filters with −3 dB cutoff frequencies between 30 and 50 kHz. The drop of 0.5 to 1.5 dB (at 20 kHz) that occurs in the upper audio range is corrected for the nominal impedance.

The structure is designed as a bridge circuit . Consistently, the abbreviation BTL (of English. B Ridge t ied l oad ). Advantages:

  • the power consumption increases roughly proportionally to the output power
  • Efficiency well over 90% at full load, 35 to 50% at 1% of the maximum output power. Comparison class B: 70 to 75% at full load, 3 to 5% at 1% of the maximum output power.

Disadvantage:

  • high driver power required
  • Construction must be based on HF criteria
  • increased component expenditure compared to A, B, A / B, C amplifiers with a discrete structure
  • LC low pass required
  • complex negative feedback (especially when driving complex loads).

A variant of the D amplifier is the T amplifier, named after the Dr. Adya S. Tripathi founded the US company Tripath Technology Inc., which patented an improved circuit topology at the end of the 1990s and brought various ICs (TA-2020, among others) onto the market. For electronic hobbyists, TAMP amplifier modules based on this circuit design are available from various manufacturers (Sure).

E amplifier

Class E amplifiers combine elements of the class D and class C amplifiers into an audio amplifier of the highest efficiency. In these, a switching stage works on a resonance circuit, the voltage of which reaches the load via a low-pass filter. The switching stage always closes when the resonant circuit has reached the zero crossing, which further reduces switching losses and interference compared to class D amplifiers. This amplifier only works in a restricted modulation range according to the aforementioned conditions; outside of this, the modulation characteristic exhibits strong non-linearities which can be compensated with the aid of complex negative feedback networks. The disadvantage here is the increased effort required to reduce self-excitation.

Advantages:

  • Reduction of switching losses in the circuit breakers compared to D operation
  • less electromagnetic interference (due to soft switching).

Disadvantage:

  • Driver power is insignificantly lower than that of the amplifier in D mode (gate charge in the FET and junction capacitance in the bipolar transistor have the main influence)
  • high non-linearity in the control characteristic (current / voltage relationship below the sine curve as a measure for the output alternating current power is currently non-linear; with D operation there is an almost linear relationship).

H amplifier

Simplified circuit of a class H current amplifier
Class H voltage boost

A class H amplifier is basically a class AB amplifier in which the supply voltage can be changed depending on the signal. The class H concept is used for power amplifiers with high output power in order to considerably reduce the voltage drop or power loss in the output transistors. Almost all high-performance PA power amplifiers (with the exception of Class D) are now designed as Class H or Class G. This can be easily recognized in the circuit diagrams by the transistors connected in series at the output stage, with graduated supply voltages, usually in two, but also in three stages.

The schematic example circuit shows the functional principle of Class H. For the sake of simplicity, only the state of the positive branch is explained below: In idle mode, T1 is supplied with +40 V via D2. The base voltage at T2 is about 10 V via D1 when the output is idle. However, the emitter potential is 40 V. Since there is no positive voltage across the BE path from T2, it blocks. When the output stage is controlled, its output voltage increases - and with it the voltage at the base of T2. At around 30 V output voltage, T2 begins to conduct and keeps the operating voltage of T1 always around 10 V above the current output voltage of the output stage. In this state, only minor losses occur at T1 due to the voltage drop limited to 10 V, but T2 is heavily loaded. The total losses in this modulation range are consequently at the level of an output stage that is generally supplied with 80 V. The savings potential of Class H therefore only takes place at instantaneous values ​​of the output voltage, where T2 is still blocked. However, this is mostly the case with music signals. For practical operation it is necessary to protect T2 against negative BE voltage by means of a diode. The quiescent current generation of the output stage is also neglected here.

The oscillogram shown shows the voltage at the cathode of D2 (channel A) and the current output voltage (channel B) of a class H power amplifier for a music signal. The function of increasing the voltage is clearly recognizable.

Power output stages from around 1 kW are often also equipped with a three-stage Class H output stage. Disadvantages of Class H are the higher circuit complexity and low, additional distortion when switching to the higher voltage level. In particular, high demands are placed on the reverse recovery behavior of the diodes, as this has a significant influence on the distortion behavior of the circuit. The negative feedback regulates jumps in the operating voltage, but not 100%. These disadvantages are partially compensated by cost advantages due to the saved heat sink size and number of output transistors.

As an extension of the classic Class H, it is also possible to generally provide the operating voltage of the output stage via a regulated buck converter. In this case, the efficiency of Class D is almost the same as that of Class AB in terms of sound properties (damping factor in the high frequency range). Examples can be found in the high-end audio sector as well as in the PA sector (Lab Groups Ferrite Power Series). The regulation of the buck converter places high demands on the dynamics, since the voltage must be adjusted quickly enough to also follow the audio signal in the high frequency range.

Class H output stages with bootstrap charge pumps are used when the operating voltages are low, in order to eliminate the voltage converter that would otherwise be required. A common field of application was, for example, the integrated output stage circuit of the type TDA1562 , which was often used in car radios . There, the supply voltage is temporarily brought from 12 V to almost 24 V for pulse peaks with the help of electrolytic capacitors and a charge pump . With minimal external wiring, up to 70 watts effectively (at 10% THD ) or 55 W at 0.5% THD can be achieved. The main advantage is the reduction in the power loss of the output stage transistors operated as switches. Similar modes of operation can be found in the vertical output stages of color television sets. The widespread type TDA8172 should be mentioned here as an example.

G amplifier

Instead of the linearly controlled increase, a switched increase is also possible. Here, a hard switch is made between the low and the high rail voltage. Instead of the Zener diodes, a comparator is used which fully switches T2 through when the output voltage of the output stage is sufficiently high. The sharp jump in the operating voltage is only partially compensated for by the negative feedback and is visible at the output of many amplifiers as a small kink in the voltage curve. Switching to the high rail voltage must be done with hysteresis in order to prevent oscillations. Compared to Class H, the transistors T2 and T4 are hardly stressed because they are used as switches. MOSFETs are mainly used in practical power amplifiers.

This technology is also often used in laboratory power supplies , whereby the tap of the transformer is switched depending on the set output voltage .

Single-ended, push-pull and bridge amplifiers

Because of the relatively high expenditure that had to be expended in the past when using electron tubes for each tube stage, only single-ended output stages with the operating point A operation were previously built . Because of the low power dissipation of the output tubes at that time (mostly less than 15 W), hardly more than 6 W output power was achieved with low efficiency. The necessary output transformer restricted the frequency range, but an adapted negative feedback in connection with the associated winding technology made it possible to keep the total distortion small over a large frequency range.

The distortion factor does not play a role in transmission output stages, which is why the single-ended principle still dominates here.

Inexpensive tubes and finally transistors made possible the more effective push-pull amplifiers with higher power. The ability to use npn and pnp transistors with almost identical characteristics created symmetrical circuits that almost completely cancel out even harmonics. By eliminating the transformer for “ironless output stages”, there was no longer a hysteresis curve , and the strong negative feedback that was now possible, which reduces distortion, considerably reduced the distortion factor. This was the beginning of HiFi.

Philips developed this "ironless power amplifier" on a tube basis in the 1950s and often used it in its own devices.

In modern tube amplifiers, this further development of quality is deliberately avoided: Often, emphasis is placed on the smallest possible negative feedback and on a certain ratio between the spectral power components of even and odd harmonics. In doing so, linearity is deliberately avoided and the rather non- linear transmission behavior of the electron tubes is used. In single-ended A operation, the transfer function results in a dominance of even-numbered harmonics and a distortion spectrum that decays very quickly towards a higher order or frequency. Due to the very low negative feedback, transient distortions are not a problem. This behavior is bought at the cost of a very low level of efficiency, consequently high heat loss and problematic magnetic saturation behavior of the output transformer, caused by the flow of the high quiescent current through the primary winding.

Circuit diagram of the H-bridge

The desire for light but powerful power amplifiers in cars with only 12 V on-board supply voltage led to the adoption of the digital bridge amplifiers known from drive technology in analog technology. The circuit diagram is reminiscent of the letter "H"; instead of the motor in the bridge arm, there is now the loudspeaker. However, the pairs AD or BC must not only be switched on and off with little loss, they have to be opened and closed slowly in finely graded steps - a lot of heat is generated and the efficiency is up to 80%, as in the usual AB operation, if with bushing Operating voltage is being worked. The maximum achievable power is four times as large as with a common "ironless power amplifier" and is calculated according to the following formula:

With a 4-ohm loudspeaker, the maximum output is 24 W. If you want more output, either the loudspeaker resistance has to be reduced, or the device has a voltage converter that converts the operating voltage to e.g. B. increases 40 V.

See also

Web links

Notes and individual references

  1. ↑ A circuit for an amplifier in class H operation and a complete explanation of how it works can be found in the magazine Elektor , issue 3/95, 1995.
  2. ↑ Radios with "ironless" tube output stage - circuit diagrams and radio types from 1955 to 1960. Wumpus Welt der Radios, welt-der-alten-radios.de.